Synthesis and characterization of (Ga_1−xZn_x)(N_1−xO_x) nanocrystals with a wide range of compositions

Abstract

We describe the synthesis and characterization of wurtzite (Ga 1Àx Zn x )(N 1Àx O x ) nanocrystals with a wide range of compositions and a focus on properties relevant for solar fuel generation. (Ga 1Àx Zn x )(N 1Àx O x ), a solid solution of GaN and ZnO, is an intriguing material because it exhibits composition-dependent visible absorption even though the parent semiconductors absorb in the UV. When functionalized with co-catalysts, (Ga 1Àx Zn x )(N 1Àx O x ) is also capable of water splitting under visible irradiation. Here, we examine the synthesis of (Ga 1Àx Zn x )(N 1Àx O x ) nanocrystals to understand how they form by nitridation of ZnO and ZnGa 2 O 4 nanocrystalline precursors. We find that the ZnO precursor is critical for the formation of crystalline (Ga 1Àx Zn x )(N 1Àx O x ) at 650 C, consistent with a mechanism in which wurtzitenucleates topotactically on wurtzite ZnO at an interface with ZnGa 2 O 4 . Using this information, we expand the range of compositions from previously reported 0.30 # x # 0.87 to include the low-x and high-x ends of the range. The resulting compositions, 0.06 # x # 0.98, constitute the widest range of (Ga 1Àx Zn x )(N 1Àx O x ) compositions obtained by one synthetic method. We then examine how the band gap depends on sample composition and find a minimum of 2.25 eV at x ¼ 0.87, corresponding to a maximum possible solar-to-H 2 power conversion efficiency of 12%. Finally, we examine the photoelectrochemical (PEC) oxidation behavior of thick films of (Ga 1Àx Zn x )(N 1Àx O x ) nanocrystals with x ¼ 0.40, 0.52, and 0.87 under visible illumination. (Ga 1Àx Zn x )(N 1Àx O x ) nanocrystals with x ¼ 0.40 exhibit solar PEC oxidation activity that, while too low for practical applications, is higher than that of bulk (Ga 1Àx Zn x )(N 1Àx O x ) of the same composition. The highest photocurrents are observed at x ¼ 0.52, even though x ¼ 0.87 absorbs more visible light, illustrating that the observed photocurrents are a result of an interplay of multiple parameters which remain to be elucidated. This set of characterizations provides information useful for future studies of composition-dependent PEC properties of nanoscale (Ga 1Àx Zn x )(N 1Àx O x ). † Electronic supplementary information (ESI) available: Fitting of XRD patterns in Fig. 2; TEM images of the samples from Fig. 2; XRD patterns, elemental analysis by ICP-OES, and diffuse reectance spectra of the products from nitridation of the starting mixture with x ¼ 0.78 with varying nitridation time; TEM images of the nitrided products of x ¼ 0.06, 0.24, 0.91, and 0.98; XPS spectra of Zn2p 3/2 , O1s, Ga2p 3/2 , and N1s in samples with several compositions; determination of band gap as a function of composition. See

Nitridation of metal salen-derived ZnGa₂O₄ particles using a solid nitrogen source

“…A mixture of the orthorombic LiGaO2 and wurtzite GaN was determined for ZG1-2 ( Figure 4c). This result suggested that the nitridation of single-phase ZnGa2O4 does not produce an oxynitride compound [2]. The loss of zinc may cause a reduction of ZnO to metallic Zn during the nitridation [1,11,16,17], suggesting that the presence of zinc oxide is required to efficiently synthsize (Ga1-xZnx)(N1-xOx).…”

“…Oxynitride semiconductor is one of the intriguing materials for energy conversion [1][2][3]. Domen et al for the first time reported on the development of GaN:ZnO solid solution for visible-light driven water splitting [4].…”

An artificial photosynthesis anode electrode composed of a nanoparticulate photocatalyst film in a visible light responsive GaN-ZnO solid solution system

“…All the photocurrents of the Ga 1-x Zn x N 1-x O x solid solutions, including the literature data reported thus far, were converted into the value of 1.23 V of an NHE at a pH of 0 and plotted. The photocurrent characteristics of Ga 1-x Zn x N 1-x O x solid solutions have been reported for films formed by the electrophoresis method12 and the doctor blade method16. The photocurrents reported thus far are on the several μA/cm 2 level.…”

“…This result led researchers to consider synthesis methods capable of further increasing the dissolved amount of ZnO. Syntheses under high pressure14, using nanoparticulate raw material1516, using a Zn/Ga/CO 3 layered double hydroxide1718, and using a ZnGaO precursor1920 were studied, and solid solutions with X = 0.8 to 0.9 were synthesized. In all of the synthesis methods, the band gap tends to decrease with the amount of ZnO added.…”

Multimetallic Oxynitrides Nanoparticles for a New Generation of Photocatalysts

“…[9] Currently,t he ZnO:GaN solid solutionsa re alwaysp repared by nitridation of the mixed oxide commonly by co-precipitation and more recently by sol-gelb ased processes;h owever, so far alwaysu nder NH 3 flow as nitrification agent. [9,10] Commons tarting materials are the single or mixed oxides, [11,12,13,14] hydroxide, [15] gel-like precursors, [16] Zn/Ga/CO 3 layered double hydroxides [17] or ammonolysis of amorphous precursors. [18] Even when urea was used as nitrogen source, the synthesis required the preparation of hydroxides, which were treated at 900 8Cu nder ammonia flow.…”